The present invention relates to the compression of digital data, and more particularly to a method and apparatus for providing motion compensation for an interlaced digital video signal, such as a television signal.
Digital transmission of television signals can deliver video and audio services of much higher quality than previous analog techniques. Digital transmission schemes are particularly advantageous for signals that are broadcast by satellite to cable television affiliates and/or directly to home satellite television receivers. Such signals can also be transmitted via a cable television network.
A substantial amount of digital data must be transmitted in any digital television system. This is particularly true where high definition television ("HDTV") is provided. In a digital television system, a subscriber receives the digital data stream via a receiver/descrambler that provides video, audio and data to the subscriber. In order to most efficiently use the available radio frequency spectrum, it is advantageous to compress the digital television signals to minimize the amount of data that must be transmitted.
The video portion of a television signal comprises a sequence of video "frames" that together provide a moving picture. In digital television systems, each line of a video frame is defined by a sequence of digital data bits referred to as "pixels." A large amount of data is required to define each video frame of a television signal. In order to manage this amount of data, particularly for HDTV applications, the data must be compressed.
Video compression techniques enable the efficient transmission of digital video signals over conventional communication channels. Such techniques use compression algorithms that take advantage of the correlation among adjacent pixels in order to derive a more efficient representation of the important information in a video signal. The most powerful compression systems not only take advantage of spacial correlation, but can also utilize similarities among adjacent frames to further compact the data. In such systems, differential encoding is used to transmit only the difference between an actual frame and a prediction of the actual frame. The prediction is based on information derived from a previous frame of the same video sequence. Examples of such systems can be found in Krause et al. U.S. Pat. Nos. 5,068,724; 5,091,782; and 5,093,720 which relate to various motion compensation schemes for digital video.
In motion compensation systems of the type disclosed in the referenced patents, motion vectors are derived by comparing a portion (i.e., block) of pixel data from a field of a current frame to similar portions of the previous frame. A motion estimator determines how the corresponding motion vector in the previous frame should be adjusted in order to be used in the current field. Such systems are very effective in reducing the amount of data to be transmitted, particularly when the source video is in a progressive scan format. However, when applied to interlaced video in which the odd and even numbered lines of a picture are transmitted consecutively as two separate interleaved fields, the efficiency of existing motion compensation techniques is sufficiently reduced.
It would be advantageous to provide a motion compensation system for interlaced digital video signals in which a greater degree of data compression can be achieved. The present invention provides such a system.